Microcapsules can be constructed of various types of wall or shell materials to house varying core material for many purposes. The encapsulation process is commonly referred to as microencapsulation. Microencapsulation is the process of surrounding or enveloping one substance, often referred to as the core material, within another substance, often referred to as the wall, shell, or capsule, on a very small scale. The scale for microcapsules may be particles with diameters in the range between 1 and 1000 μm that consist of a core material and a covering shell. The microcapsules may be spherically shaped, with a continuous wall surrounding the core, while others may be asymmetrical and variably shaped.
General encapsulation processes include emulsion polymerization, bulk polymerization, solution polymerization, and/or suspension polymerization and typically include a catalyst. Emulsion polymerization occurs in a water/oil or oil/water mixed phase. Bulk polymerization is carried out in the absence of solvent. Solution polymerization is carried out in a solvent in which both the monomer and subsequent polymer are soluble. Suspension polymerization is carried out in the presence of a solvent (usually water) in which the monomer is insoluble and in which it is suspended by agitation. To prevent the droplets of monomers from coalescing and to prevent the polymer from coagulating, protective colloids are typically added.
Through a selection of the core and shell material, it is possible to obtain microcapsules with a variety of functions. This is why microcapsules can be defined as containers, which can release, protect and/or mask various kinds of active core materials. Microencapsulation is mainly used for the separation of the core material from the environment, but it can also be used for controlled release of core material in the environment. Microencapsulation has attracted a large interest in the field of phase change materials (PCMs). A PCM is a substance with a high heat of fusion, melting and solidifying at a certain temperature, which is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage units. The latent heat storage can be achieved through solid-solid, solid-liquid, solid-gas and liquid-gas phase change, but solid-liquid is typically used in thermal storage applications as being more stable than gas phase changes as a result of the significant change in volume occupied by the PCM.
Melamine-formaldehyde resin is often used as the shell material for encapsulating PCMs because of its good mechanical and thermal stability. Melamine-formaldehyde (MF) microcapsules can be prepared by the in situ polymerization process of polycondensation, where the melamine-formaldehyde prepolymer is initially soluble in the continuous water phase, while the hydrophobic core material is contained in dispersed droplets. As the polymerization reaction starts in the aqueous solution, the formed oligomers start to collapse on the surface of the core droplets. On the surface, the polymerization continues and crosslinking occurs, which results in a solid MF shell formation. The shell formation and the amount of free formaldehyde present in the capsule have been shown to be related to various factors such as the pH, temperature, type and the amount of emulsifier, and the molar ratio of melamine formaldehyde to emulsifier, which all affect thermal stability, shell morphology and remnant free formaldehyde.
The term “free formaldehyde” means those molecular forms present in aqueous solution capable of rapid equilibration with the native molecule, in the headspace over the solution. This includes the aqueous native molecule, its hydrated form methanediol and its polymerized hydrated form (HO(CH2O). Methanediol is a product of the hydration of formaldehyde H2C═O, and predominates in water solution: the equilibrium constant being about 103, and in a 5% by weight solution of formaldehyde in water, 80% is in the methanediol form.
Free formaldehyde is generated as a byproduct of the melamine-formaldehyde poly-condensation reaction and/or from a hydrolytic attack on the melamine formaldehyde polymer. The melamine formaldehyde polymer can continuously release formaldehyde under moist and acidic conditions; thus, free formaldehyde levels may increase over time due to residual curing, and hydrolysis of the end-groups, in the cross-linked microcapsule wall. In addition, the unreacted residual formaldehyde enclosed within the microcapsules can also be released via simple diffusion.
Attempts have been made to reduce remnant free formaldehyde. For example, formaldehyde scavengers have been introduced in one attempt to lower free formaldehyde levels. Urea has been shown to be an effective formaldehyde scavenger. It is believed that as well as being a formaldehyde scavenger, urea is able to undergo a cross-linking reaction with the polymeric wall of the microcapsules, and inhibit the release of free formaldehyde from the microcapsule wall. Hence, it is believed that urea can both reduce the generation of free formaldehyde, and scavenge any formaldehyde that is released into a slurry or composition. For instance, when the microcapsule wall is formed by cross-linking formaldehyde with melamine, it is believed that urea is able to react with the methylol groups of the melamine-formaldehyde polymeric wall, and inhibits the release of free formaldehyde from the microcapsule wall. Moreover, when the urea complexes with the microcapsule wall, particularly walls made from crosslinking urea, melamine, and mixtures thereof with formaldehyde, the wall is made less porous. As a consequence, leakage of the raw materials from the microcapsule core is reduced. When urea is used, the urea is preferably added directly to the microcapsule slurry. When urea is first added to the microcapsule slurry, a pH of less than 5.5 is particularly preferred for the microcapsule slurry, for improved formaldehyde scavenging and microcapsule wall stability.
Even with use of the most efficient formaldehyde scavengers, such as urea, there still exists the need for free formaldehyde reduction. In particular, reducing free formaldehyde from both interstitially bonded formaldehyde and formaldehyde due to hydrolysis (hydrolytic attack) is desirable. FIG. 1 shows both the initial amount of free formaldehyde present for a melamine urea formaldehyde microcapsule having a PCM core material. On Day 1, about 200 ppm of free formaldehyde for a wet cake was present at the beginning of the process, and the amount of formaldehyde rose over time, due to hydrolysis, and eventually fell after about Days 7-12, depending upon the drying method, as the hydrolysis process slowed and the formaldehyde which is volatile was eliminated.
Since the development of microencapsulated PCMs, there has been a constant need for improved microcapsules; in particular, there is a need for reductions in remnant free formaldehyde when melamine formaldehyde resins form the shell of the microcapsules, while maintaining acceptable thermostability and enthalpy values.